1. Field of the Invention
The present invention relates to water injection type screw fluid machines.
2. Description of the Related Art
In a screw fluid machine, such as a screw compressor for compressing a target gas by means of intermeshing male and female screw rotors housed in a rotor chamber, or a screw expander (an expansion apparatus), in which the target gas is expanded to rotate the intermeshing male and female screw rotors in the rotor chamber, a shaft sealing structure is provided between a rotor shaft of the screw rotor and a bearing to seal the target gas in a system or prevent the target gas from being mixed with outside air or the like.
In conventional screw compressors as described in Japan Patent No. 4559343, a lip seal is used as a shaft sealing device on the intake side, while a mechanical seal is used as a shaft sealing device on the discharge side.
Although the lip seal is an inexpensive and space saving shaft sealing device, the maximum pressure which can be sealed by the lip seal is typically around 0.3 kgf/cm2. For this reason, since the lip seal could have an insufficient shaft sealing effect or tend to be significantly inferior in durability when used on the high pressure side, the lip seal can be only used to seat a shaft on the low pressure side. On the other hand, the mechanical seal, which is capable of sealing a shaft under on the high pressure side, is problematic in terms of its extremely high cost and large footprint.
In the screw compressor disclosed in the above noted Japan Patent No. 4559343, the lip seal is used for sealing a shaft on the intake side and also used for sealing the shaft on the discharge side. In order to prevent application of an excessive pressure on the lip seal, which is used for sealing the shaft on the discharge side, the screw compressor is equipped with a labyrinth seal disposed between a screw rotor and the lip seal, and a communicating channel for allowing a space between the labyrinth seal and the lip seal to be communicated with an intake channel or an intermediate pressure section located close to the intake side in the rotor chamber.
On the other hand, as described in JP 2000-45948-A, for example, some screw fluid machines are of a water injection type that water is injected into a rotor chamber for the purposes of lubrication and cooling. When the lip seal is used as the shaft sealing device in such a screw fluid machine of the water injection type, it is necessary for the lip seal to have a water sealing function. However, because lubrication property of water is poor as contrasted to oil, the lip seal becomes more vulnerable to abrasion when it is used for sealing water. Therefore, such a water injection type screw fluid machine suffers from a problem that the lip seal has a short service life, necessitating frequent maintenance.